The muscarinic receptor gene expressed in rabbit parietal cells is the m3 subtype

Muscarinic Receptors Associated with Cancer

Simple Summary

Recently, cancer research has described the presence of the cholinergic machinery, specifically muscarinic receptors, in a wide variety of cancers due to their activation and signaling pathways associated with tumor progression and metastasis, providing a wide overview of their contribution to different cancer formation and development for new antitumor targets. This review focused on determining the molecular signatures associated with muscarinic receptors in breast and other cancers and the need for pharmacological, molecular, biochemical, technological, and clinical approaches to improve new therapeutic targets.

Abstract

Cancer has been considered the pathology of the century and factors such as the environment may play an important etiological role. The ability of muscarinic agonists to stimulate growth and muscarinic receptor antagonists to inhibit tumor growth has been demonstrated for breast, melanoma, lung, gastric, colon, pancreatic, ovarian, prostate, and brain cancer. This work aimed to study the correlation between epidermal growth factor receptors and cholinergic muscarinic receptors, the survival differences adjusted by the stage clinical factor, and the association between gene expression and immune infiltration level in breast, lung, stomach, colon, liver, prostate, and glioblastoma human cancers. Thus, targeting cholinergic muscarinic receptors appears to be an attractive therapeutic alternative due to the complex signaling pathways involved.

Role of Muscarinic Acetylcholine Signaling in Gastrointestinal Cancers

In the tumor microenvironment, various stromal and immune cells accumulate and interact with cancer cells to contribute to tumor progression. Among stromal players, nerves have recently been recognized as key regulators of tumor growth. More neurotransmitters, such as catecholamines and acetylcholine (ACh), are present in tumors, as the cells that secrete neurotransmitters accumulate by the release of neurotrophic factors from cancer cells. In this short review, we focus on the role of nerve signaling in gastrointestinal (GI) cancers. Given that muscarinic acetylcholine receptor signaling seems to be a dominant regulator of GI stem cells and cancers, we review the function and mechanism of the muscarinic ACh pathway as a regulator of GI cancer progression. Accumulating evidence suggests that ACh, which is secreted from nerves and tuft cells, stimulates GI epithelial stem cells and contributes to cancer progression via muscarinic receptors.

Adenosine: Direct and Indirect Actions on Gastric Acid Secretion

Composed by a molecule of adenine and a molecule of ribose, adenosine is a paradigm of recyclable nucleoside with a multiplicity of functions that occupies a privileged position in the metabolic and regulatory contexts. Adenosine is formed continuously in intracellular and extracellular locations of all tissues. Extracellular adenosine is a signaling molecule, able to modulate a vast range of physiologic responses in many cells and organs, including digestive organs. The adenosine A1, A2A, A2B, and A3 receptors are P1 purinergic receptors, G protein-coupled proteins implicated in tissue protection. This review is focused on gastric acid secretion, a process centered on the parietal cell of the stomach, which contains large amounts of H⁺/K⁺-ATPase, the proton pump responsible for proton extrusion during acid secretion. Gastric acid secretion is regulated by an extensive collection of neural stimuli and endocrine and paracrine agents, which act either directly at membrane receptors of the parietal cell or indirectly through other regulatory cells of the gastric mucosa, as well as mechanic and chemic stimuli. In this review, after briefly introducing these points, we condense the current body of knowledge about the modulating action of adenosine on the pathophysiology of gastric acid secretion and update its significance based on recent findings in gastric mucosa and parietal cells in humans and animal models.

Activation of Muscarinic Acetylcholine Receptor Subtype 4 Is Essential for Cholinergic Stimulation of Gastric Acid Secretion: Relation to D Cell/Somatostatin

Background/Aim: Muscarinic acetylcholine receptors exist in five subtypes (M₁∼M₅), and they are widely expressed in various tissues to mediate diverse autonomic functions, including gastric secretion. In the present study, we demonstrated, using M1∼M5 KO mice, the importance of M4 receptors in carbachol (CCh) stimulation of acid secretion and investigated how the secretion is modulated by the activation of M4 receptors.

Methods: C57BL/6J mice of wild-type (WT) and M1–M5 KO were used. Under urethane anesthesia, acid secretion was measured in the stomach equipped with an acute fistula. CCh (30 μg/kg) was given subcutaneously (s.c.) to stimulate acid secretion. Atropine or octreotide (a somatostatin analog) was given s.c. 20 min before the administration of CCh. CYN154806 (a somatostatin SST2 receptor antagonist) was given i.p. 20 min before the administration of octreotide or CCh.

Results: CCh caused an increase of acid secretion in WT mice, and the effect was totally inhibited by prior administration of atropine. The effect of CCh was similarly observed in the animals lacking M1, M2 or M5 receptors but significantly decreased in M3 or M4 KO mice. CYN154806, the SST2 receptor antagonist, dose-dependently and significantly reversed the decreased acid response to CCh in M4 but not M3 KO mice. Octreotide, the somatostatin analog, inhibited the secretion of acid under CCh-stimulated conditions in WT mice. The immunohistochemical study showed the localization of M₄ receptors on D cells in the stomach. Serum somatostatin levels in M4 KO mice were higher than WT mice under basal conditions, while those in WT mice were significantly decreased in response to CCh.

Conclusions: These results suggest that under cholinergic stimulation the acid secretion is directly mediated by M3 receptors and indirectly modified by M4 receptors. It is assumed that the activation of M4 receptors inhibits the release of somatostatin from D cells and minimizes the acid inhibitory effect of somatostatin through SST2 receptors, resulting in enhancement of the acid response mediated by M3 receptors on parietal cells.

H. pylori acutely inhibits gastric secretion by activating CGRP sensory neurons coupled to stimulation of somatostatin and inhibition of histamine secretion

Acute Helicobacter pylori infection produces hypochlorhydria. The decrease in acid facilitates survival of the bacterium and its colonization of the stomach. The present study was designed to identify the pathways in oxyntic mucosa by which acute H. pylori infection inhibits acid secretion. In rat fundic sheets in an Ussing chamber, perfusion of the luminal surface with H. pylori in spent broth (10(3)-10(8) cfu/ml) or spent broth alone (1:10(5) to 1:10(0) final dilution) caused a concentration-dependent increase in somatostatin (SST; maximal: 200 ± 20 and 194 ± 9% above basal; P < 0.001) and decrease in histamine secretion (maximal: 45 ± 5 and 48 ± 2% below basal; P < 0.001); the latter was abolished by SST antibody, implying that changes in histamine secretion reflected changes in SST secretion. Both responses were abolished by the axonal blocker tetrodotoxin (TTX), the sensory neurotoxin capsaicin, or the CGRP antagonist CGRP8-37, implying that the reciprocal changes in SST and histamine secretion were due to release of CGRP from sensory neurons. In isolated rabbit oxyntic glands, H. pylori inhibited basal and histamine-stimulated acid secretion in a concentration-dependent manner; the responses were not affected by TTX or SST antibody, implying that H. pylori can directly inhibit parietal cell function. In conclusion, acute administration of H. pylori is capable of inhibiting acid secretion directly as well as indirectly by activating intramural CGRP sensory neurons coupled to stimulation of SST and inhibition of histamine secretion. Activation of neural pathways provides one explanation as to how initial patchy colonization of the superficial gastric mucosa by H. pylori can acutely inhibit acid secretion.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

The continuing development of gastric acid pump inhibitors

The synthesis and action of H2-receptor antagonists changed the understanding of gastric acid secretion as well as changing medical therapy for peptic ulcer disease. It is now known that peripheral regulation of gastric acid secretion depends largely, but not entirely, on histamine release from the enterochromaffin-like cell. There is, therefore, no final common pathway for stimulation of the parietal cell. In contrast, all stimuli converge to activate the acid pump, the H+,K(+)-ATPase. Inhibition of this pump by clinically useful drugs was achieved by developing derivatives of timoprazole, pyridyl-2-methylsulfinyl benzimidazole. Two of these derivatives, omeprazole and lansoprazole, have shown superiority in acid control and therefore in therapy for peptic ulcer disease compared to the available H2-receptor antagonists.

Necessity of intracellular cyclic AMP in inducing gastric acid secretion via muscarinic M3 and cholecystokinin2 receptors on parietal cells in isolated mouse stomach

The existence of a direct action of acetylcholine and gastrin on muscarinic M3 and cholecystokinin2 (CCK2) receptors on gastric parietal cells has not yet been convincingly established because these stimulated acid secretions are remarkably inhibited by histamine H2 receptor antagonists. In the present study, we investigated the necessity of intracellular cyclic AMP in inducing gastric acid secretion via muscarinic M3 and CCK2 receptors on parietal cells using an isolated mouse stomach preparation. Bethanechol (10-300 microM) produced a marked increase in acid output and this increase was completely blocked by famotidine (10 microM). In the presence of famotidine, bethanechol (1-30 microM) augmented the acid secretory response to dibutyryl AMP (200 microM) in a concentration-dependent manner. The augmentation was blocked by atropine (1 microM), 4-DAMP (0.1 microM), a muscarinic M3-selective antagonist, and by Ca2+ exclusion from the serosal nutrient solution. Pentagastrin (0.3-3 microM) also concentration-dependently stimulated gastric acid secretion, but the effect was completely inhibited by famotidine. In the presence of famotidine, pentagastrin (0.1-0.3 microM) elicited a definite potentiation of the acid secretory response to dibutyryl cyclic AMP (200 microM). This potentiation was inhibited by YM022 (1 microM), a CCK2 receptor antagonist, and by exclusion of Ca2+ from the serosal nutrient solution. The present results suggest that gastric acid secretion via the activation of muscarinic M3 and CCK2 receptors on the parietal cells is induced by activation of the cyclic AMP-dependent secretory pathway.

Cholinergic agonist-induced pepsinogen secretion from murine gastric chief cells is mediated by M1 and M3 muscarinic receptors

Muscarinic cholinergic mechanisms play a key role in stimulating gastric pepsinogen secretion. Studies using antagonists suggested that the M3 receptor subtype (M3R) plays a prominent role in mediating pepsinogen secretion, but in situ hybridization indicated expression of M1 receptor (M1R) in rat chief cells. We used mice that were deficient in either the M1 (M1R-/-) or M3 (M3R-/-) receptor or that lacked both receptors (M(1/3)R-/-) to determine the role of M1R and M3R in mediating cholinergic agonist-induced pepsinogen secretion. Pepsinogen secretion from murine gastric glands was determined by adapting methods used for rabbit and rat stomach. In wild-type (WT) mice, maximal concentrations of carbachol and CCK caused a 3.0- and 2.5-fold increase in pepsinogen secretion, respectively. Maximal carbachol-induced secretion from M1R-/- mouse gastric glands was decreased by 25%. In contrast, there was only a slight decrease in carbachol potency and no change in efficacy when comparing M3R-/- with WT glands. To explore the possibility that both M1R and M3R are involved in carbachol-mediated pepsinogen secretion, we examined secretion from glands prepared from M(1/3)R-/- double-knockout mice. Strikingly, carbachol-induced pepsinogen secretion was nearly abolished in glands from M(1/3)R-/- mice, whereas CCK-induced secretion was not altered. In situ hybridization for murine M1R and M3R mRNA in gastric mucosa from WT mice revealed abundant signals for both receptor subtypes in the cytoplasm of chief cells. These data clearly indicate that, in gastric chief cells, a mixture of M1 and M3 receptors mediates cholinergic stimulation of pepsinogen secretion and that no other muscarinic receptor subtypes are involved in this activity. The development of a murine secretory model facilitates use of transgenic mice to investigate the regulation of pepsinogen secretion.

Cholinergically stimulated gastric acid secretion is mediated by M(3) and M(5) but not M(1) muscarinic acetylcholine receptors in mice

Muscarinic acetylcholine receptors play an important role in the regulation of gastric acid secretion stimulated by acetylcholine; nonetheless, the precise role of each receptor subtype (M(1)-M(5)) remains unclear. This study examined the involvement of M(1), M(3), and M(5) receptors in cholinergic regulation of acid secretion using muscarinic receptor knockout (KO) mice. Gastric acid secretion was measured in both mice subjected to acute gastric fistula production under urethane anesthesia and conscious mice that had previously undergone pylorus ligation. M(3) KO mice exhibited impaired gastric acid secretion in response to carbachol. Unexpectedly, M(1) KO mice exhibited normal intragastric pH, serum gastrin and mucosal histamine levels, and gastric acid secretion stimulated by carbachol, histamine, and gastrin. Pirenzepine, known as an M(1)-receptor antagonist, inhibited carbachol-stimulated gastric acid secretion in a dose-dependent manner in M(1) KO mice as well as in wild-type (WT) mice, suggesting that the inhibitory effect of pirenzepine on gastric acid secretion is independent of M(1)-receptor antagonism. Notably, M(5) KO mice exhibited both significantly lower carbachol-stimulated gastric acid secretion and histamine-secretory responses to carbachol compared with WT mice. RT-PCR analysis revealed M(5)-mRNA expression in the stomach, but not in either the fundic or antral mucosa. Consequently, cholinergic stimulation of gastric acid secretion is clearly mediated by M(3) (on parietal cells) and M(5) receptors (conceivably in the submucosal plexus), but not M(1) receptors.

Regulation of expression of the receptors controlling gastric acidity

Gastric acid secretion is regulated by the stimulatory effects of gastrin, histamine and acetylcholine and the inhibitory actions of somatostatin on their respective receptors. We proposed that the expression of these receptors could be regulated at the transcription level by agonists and antagonists known to effect acid secretion. A quantitative "real-time" PCR method was used to determine changes in mRNA expression for these receptors. The agonists, pentagastrin and histamine, and the H2 antagonist, ranitidine, were infused over a 6 h period to conscious sheep. Blood, antral and fundic tissue samples were taken for analysis. Both pentagastrin and histamine resulted in elevated plasma somatostatin concentrations during the treatment. Ranitidine stimulated a fourfold increase in plasma gastrin while histamine caused a transient decrease. Except for an increase in antral gastrin following ranitidine infusion, there was no significant change in gastric gastrin and somatostatin concentration. Histamine (H2) receptor mRNA expression in the antrum was significantly increased by pentagastrin and decreased by ranitidine. Pentagastrin also stimulated a significant increase in the level of muscarinic (M3) receptor mRNA in the antrum. Antral somatostatin II receptor mRNA was significantly decreased by histamine. In the fundus, pentagastrin infusion resulted in a significant increase in histamine receptor mRNA and a decrease in the muscarinic receptor mRNA. This work demonstrates that the receptors involved in the regulation of acid secretion can be regulated by local events.

New molecular targets for treatment of peptic ulcer disease

Most patients with peptic ulcer disease are currently treated with proton pump inhibitors or histamine H(2) receptor antagonists. The long-term use of these compounds has been associated with two potential problems. Firstly, proton pump inhibitors may induce enterochromaffin-like (ECL) cell hyperplasia. Secondly, ulcers may relapse despite maintenance therapy with histamine H(2) antagonists. This has been the rationale for the development of new antisecretory agents, including antagonists against gastrin and gastrin releasing peptide (GRP), as well as ligands to histamine H(3) receptors. Several potent, high affinity cholecystokinin (CCK)-2 receptor antagonists have recently been identified such as L-365260, YM-022, RP-73870, S-0509, spiroglumide and itriglumide (CR-2945). Current data suggest that they all have antisecretory and anti-ulcer effects. In addition to reducing acid production, CCK-2 receptor antagonists may possibly also accelerate gastric emptying, a combination of functions which could potentially be beneficial in patients with functional dyspepsia. Receptors for bombesin and its mammalian counterpart GRP have been localised in the brain, spinal cord and enteric nerve fibres of the gut as well as on secretory cells and smooth muscle cells of the intestinal tract. Current data clearly indicate that endogenous GRP is involved in the regulation of basal and postprandial acid secretion. However, at this stage it is not clear whether GRP agonists or GRP antagonists can be developed into useful drugs. The peptide has a wide range of biological effects and it is likely that analogues of GRP or antagonists of the peptide affect not only gastric acid secretion but also induce considerable side effects. Histamine plays a central role in the stimulation of acid secretion. After their detection in the brain, H(3) receptors have been identified in a variety of tissues including perivascular nerve terminals, enteric ganglia of the ileum and lung, and ECL cells. Despite many studies, the role of H(3) receptors in the regulation of gastric acid secretion is still unclear. Controversial data have been presented, and study results largely depend on the species and experimental models. It seems unlikely that proton pump inhibitors or H(2) receptor antagonists will be replaced in the near future by new antisecretory agents. The current shortcomings of the new compounds include mainly their reduced clinical effectiveness and pharmacological limitations. However, the development of these new antisecretory compounds provides interesting tools to assess the physiological and pharmacological role of different receptors within the gastrointestinal tract. The use of CCK-2 receptor antagonists in patients with functional dyspepsia and Zollinger-Ellison syndrome should be examined in randomised, controlled trials.

Impaired gastric secretion and lack of trophic responses to hypergastrinemia in M3 muscarinic receptor knockout mice

BACKGROUND & AIMS

The physiologic significance of the M(3) muscarinic receptor is unclear due to an absence of specific ligand. In the present study, M(3) receptor knockout (KO) mice were used to elucidate the role of M(3) receptors in gastric acid secretion and gastric mucosal integrity.

METHODS

M(3) KO versus wild-type mice aged 1 month to 2 years were included. Gastric acid secretion was assessed by both direct intragastric pH measurement and pylorus ligation. Serum gastrin and gastric mucosal histamine levels were determined by radioimmunoassay and enzyme-linked immunosorbent assay, respectively. Morphologic analysis was performed by both immunohistochemistry and transmission electron microscopy.

RESULTS

Fasted M(3) KO mice exhibited higher intragastric pH, lower acid output after pylorus ligation, a lower proportion of active parietal cells, and higher serum gastrin levels than fasted wild-type mice. Acid secretion in response to carbachol, histamine, gastrin 17, and 2-deoxy-D-glucose was impaired in the mutant mice. Although carbachol was still able to cause approximately 30% acid output in M(3) KO mice, the acid secretion was inhibited by pirenzepine or famotidine. Despite remarkable hypergastrinemia in M(3) KO mice, there were no trophic responses in the oxyntic mucosa with respect to the mucosal thickness, proliferation rate, and numbers of parietal and enterochromaffin-like cells. Cholecystokinin type 2 receptor antagonist YM022 was without the effect in M(3) KO mice.

CONCLUSIONS

The present study shows that M(3) receptors are essential for basal acid secretion, a fully acid secretory response to histamine and gastrin, and the trophic responses of oxyntic mucosa to gastrin.

Protein kinase C-alpha attenuates cholinergically stimulated gastric acid secretion of rabbit parietal cells

(1) The phorbolester 12-O-tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion. We observed that this effect strongly correlated with the inhibition of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity in rabbit parietal cells. (2) The aim of this study was to specify the function of PKC-alpha in cholinergically stimulated H(+) secretion. PKC-alpha represents the only calcium-dependent PKC isoenzyme that has been detected in rabbit parietal cells. (3) Gö 6976, an inhibitor of calcium-dependent PKC, concentration-dependently antagonized the inhibitory effect of TPA, and, therefore, revealed the action of PKC-alpha on carbachol-induced acid secretion in rabbit parietal cells. (4) TPA exerted no additive inhibition of carbachol-stimulated acid secretion if acid secretion was partially inhibited by the potent CaMKII inhibitor 1-[N,O-bis(5-isoquinolinsulfonyl)-N-methyl-L-tyrosyl]-4-phenyl-piperazine (KN-62). (5) Since both kinase modulators, TPA and KN-62, affected no divergent signal transduction pathways in the parietal cell, an in vitro model has been used to study if PKC directly targets CaMKII. CaMKII purified from parietal cell-containing gastric mucosa of pig, was transphosphorylated by purified cPKC containing PKC-alpha up to 1.8 mol P(i) per mol CaMKII in vitro. The autonomy site of CaMKII was not transphosphorylated by PKC. (6) The phosphotransferase activity of the purified CaMKII was in vitro inhibited after transphosphorylation by PKC if calmodulin was absent during transphosphorylation. Attenuation of CaMKII activity by PKC showed strong similarity to the downregulation of CaMKII by basal autophosphorylation. (7) Our results suggest that PKC-alpha and CaMKII are closely functionally linked in a cholinergically induced signalling pathway in rabbit parietal cells. We assume that in cholinergically stimulated parietal cells PKC-alpha transinhibits CaMKII activity, resulting in an attenuation of acid secretion.

Selective interaction of bile acids with muscarinic receptors: a case of molecular mimicry

Bile acids alter regulatory pathways in several cell types. The molecular basis for these actions is not fully elucidated, but lithocholyltaurine interacts functionally with muscarinic receptors on gastric chief cells. In the present report, we demonstrate selective interaction of bile acids with Chinese hamster ovary (CHO) cells expressing each of the five muscarinic receptors. Lithocholyltaurine decreases binding of a radioligand to muscarinic M3 receptors, but not to other muscarinic receptors. Sulfated lithocholyltaurine, the major human metabolite, inhibits radioligand binding to muscarinic M1, but not to M2 or M3 receptors. Post-receptor actions of lithocholyltaurine include modulation of acetylcholine-induced increases in inositol phosphate formation and mitogen-activated protein (MAP) kinase phosphorylation. Molecular modeling suggests that the specific and functional interaction of lithocholyltaurine with muscarinic receptors is most likely due to similar shape and surface charge distribution of portions of acetylcholine and the bile acid. We propose that bile acids are signaling molecules whose effects may be mediated by interaction with muscarinic receptors.

The cellular localization of the cholecystokinin 2 (gastrin) receptor in the stomach

The role of the gastric acid secretagogues acetylcholine, gastrin and histamine has been debated for decades. Initially, the mast cell was considered the source of acid stimulatory histamine. Later, Håkanson & Owman (1969) showed that the entero-chromaffinlike (ECL) cell produces and stores histamine in several species, including rat and man. Kahlson et al. (1964) showed that food and gastrin stimulated oxyntic mucosal histamine synthesis and release, Berglindh et at. (1976) that histamine and cholinergics but not gastrin induced acid secretion in isolated oxyntic glands and parietal cells, and Rangachari (1995) that acetylcholine or gastrin released histamine in isolated mucosa. These findings suggested that gastrin stimulates acid secretion through release of ECL cell histamine. Studying simultaneous histamine release and acid secretion in isolated oxyntic mucosal cells, we found that gastrin stimulated acid secretion only in preparations releasing histamine. Moreover, in the isolated rat stomach, gastrin stimulated both histamine release and acid secretion. Maximal acid output was higher with histamine than with gastrin, and augmented by acetylcholine but not by gastrin. These findings strongly suggested that gastrin acts by releasing histamine. Finally, a fluorescein-labelled gastrin analogue bound to the ECL cell, not to the parietal or stem cell regions. This is interesting, recalling that gastrin has a potent and specific trophic effect on the ECL cell and only a general effect on all other oxyntic cell types. In conclusion, physiological observations are best explained by localising the CCK2 receptor only to the ECL cell, the other effects of gastrin on the gastric mucosa being secondary to the release of mediators from the ECL cell.

Functional interaction of lithocholic acid conjugates with M3 muscarinic receptors on a human colon cancer cell line

Lithocholic acid (LA) conjugates interact with M3 receptors, the muscarinic receptor subtype that modulates colon cancer cell proliferation. This observation prompted us to examine the action of bile acids on two human colon cancer cell lines: H508, which expresses M3 receptors, and SNU-C4, which does not. Cellular proliferation was determined using a colorimetric assay. Interaction with muscarinic receptors was determined by measuring inhibition of muscarinic radioligand binding and changes in cellular inositol phosphate (IP) formation. Lithocholyltaurine (LCT) caused a dose-dependent increase in H508 cell proliferation that was not observed in SNU-C4 cells. After a 6-day incubation with 300 microM LCT, H508 cell proliferation increased by 200% compared to control. Moreover, in H508 cells, LCT caused a dose-dependent inhibition of radioligand binding and an increase in IP formation. LCT did not alter the rate of apoptosis in H508 or SNU-C4 cells. These data indicate that, at concentrations achievable in the gut, LA derivatives interact with M3 muscarinic receptors on H508 human colon cancer cells, thereby causing an increase in IP formation and cell proliferation. This suggests a mechanism whereby alterations in intestinal bile acids may affect the growth of colon cancer cells.

Lithocholylcholine, a bile acid/acetylcholine hybrid, is a muscarinic receptor antagonist

Previous work from our laboratory indicates that bile acids, specifically lithocholic acid conjugates, interact with muscarinic receptors on gastric chief cells. Structural similarities between acetylcholine and lithocholyltaurine suggest a potential molecular basis for their interaction with the same receptor. We synthesized a hybrid molecule consisting of the steroid nucleus of lithocholyltaurine and the choline moiety of acetylcholine. The new molecule, lithocholylcholine, is hydrolyzed by acetyl-cholinesterase. Lithocholylcholine inhibited binding of a cholinergic radioligand to Chinese hamster ovary cells expressing each of the five muscarinic receptor subtypes. The binding affinities (K(i); micromolar) of lithocholylcholine for these receptors were: M3 (1.0) > M1 (2.7) > M2 (4.1) = M4 (4.9) > M5 (6.2). Lithocholylcholine inhibited intracellular signaling pathways mediated by interaction with M1, M2, and M3 muscarinic receptors. Regarding M3 receptors, lithocholylcholine was 10-fold more potent than lithocholyltaurine in terms of binding affinity and inhibition of acetylcholine-induced increases in inositol phosphate formation and mitogen-activated protein kinase phosphorylation. In a functional assay, lithocholylcholine inhibited acetylcholine-induced relaxation of rat aortic rings. These observations indicate that lithocholylcholine is a muscarinic receptor antagonist and provide further evidence that bile acids may have gastrointestinal signaling functions that extend beyond their effects on sterol metabolism, lipid absorption, and cholesterol elimination. Hybrid molecules created from bile acids and acetylcholine may be used to develop selective muscarinic receptor ligands.

Different actions of protein kinase C isoforms alpha and epsilon on gastric acid secretion

1. The phorbol ester TPA, an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion but increases basal H(+) secretion. 2. Since these contradictory findings suggest the action of different PKC isozymes we analysed the role of calcium-dependent PKC-alpha, and calcium-independent PKC-epsilon in gastric acid secretion. 3. Inhibition of PKC-alpha by the indolocarbazole Gö 6976 revealed that about 28% of carbachol-induced acid secretion was inhibited by PKC-alpha. In the presence of Gö 6976 approximately 64% of the carbachol-induced signal transduction is mediated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and 14% is conveyed by PKC-epsilon as deduced from the inhibition with the bisindolylmaleimide Ro 31-8220. 4. Inhibition of carbachol-induced acid secretion by TPA was accompanied by a decrease in CaMKII activity. 5. The stimulation of basal acid secretion by TPA was biphasic with a peak at a very low concentration (10 pM), resulting in an activation of the calcium-sensor CaMKII. The activation was determined with a phosphospecific polyclonal antibody against active CaMKII. The TPA-induced increase of H(+) secretion was sensitive to the cell-permeable Ca(2+)-chelator BAPTA/AM, Ro 31-8220, and the CaMKII-inhibitor KN-62, but not to Gö 6976. 6. Since TPA induced the translocation of PKC-epsilon but not of PKC-alpha in resting parietal cells, PKC-epsilon seems to be at least responsible for an initial elevation of free intracellular calcium to initiate TPA-induced acid secretion. 7. Our data indicate the different roles of two PKC isoforms: PKC-epsilon activation appears to facilitate cholinergic stimulation of H(+)-secretion likely by increasing intracellular calcium. In contrast, PKC-alpha activation attenuates acid secretion accompanied by a down-regulation of CaMKII activity.

CaMKII is activated and translocated to the secretory apical membrane during cholinergically conveyed gastric acid secretion

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is thought to be activated during the cholinergic stimulation of gastric acid secretion. The carbachol-induced acid production of cultured rabbit parietal cells was dose-dependently inhibited by the CaMKII inhibitor KN-62 as measured by accumulation of the weak base [(14)C]aminopyrine ([(14)C]-AP). Inhibition by KN-62 was most efficient at concentrations of carbachol >10(-6) M. After carbachol stimulation, we observed an activation of CaMKII activity, and its translocation to the apical membrane of gastric mucosal cells. We found a doubling of the abundance of CaMKII to the stimulus-associated apical membrane (SA vesicles) compared to the apical membrane from the resting state after carbachol induction. This was shown by both an anti-CaMKII serum and the 1.8-fold increase of the CaMKII phosphotransferase activity in vitro. The SA vesicles exhibited a strong increase of autoactivated CaMKII probed with an anti-autoactivated CaMKII antibody. Additionally, we observed a colocalization of both CaMKII and the H(+)-K(+)-ATPase of SA vesicles similar to the colocalization of both enzymes to the tubulovesicles suggesting them as at least one pool for the SA vesicular CaMKII. Our data indicate that the activation of CaMKII and the carbachol-dependent redistribution of CaMKII to the SA vesicles are distinct processes that occur in parallel to regulate the activity and localization of CaMKII. These findings contribute to the model implicating an involvement for CaMKII in the intracellular dynamics of the acid secretion.

PACAP stimulates gastric acid secretion in the rat by inducing histamine release

Previous studies have shown that pituitary adenylate cyclase-activating peptide (PACAP) stimulates enterochromaffin-like (ECL) cell histamine release, but its role in the regulation of gastric acid secretion is disputed. This work examines the effect of PACAP-38 on aminopyrine uptake in enriched rat parietal cells and on histamine release and acid secretion in the isolated vascularly perfused rat stomach and the role of PACAP in vagally (2-deoxyglucose) stimulated acid secretion in the awake rat. PACAP has no direct effect on the isolated parietal cell as assessed by aminopyrine uptake. PACAP induces a concentration-dependent histamine release and acid secretion in the isolated stomach, and its effect on histamine release is additive to gastrin. The histamine H2 antagonist ranitidine potently inhibits PACAP-stimulated acid secretion without affecting histamine release. Vagally stimulated acid secretion is partially inhibited by a PACAP antagonist. The results from the present study strongly suggest that PACAP plays an important role in the neurohumoral regulation of gastric acid secretion. Its effect seems to be mediated by the release of ECL cell histamine.

Copurification of two holoenzyme-forming Calcium/Calmodulin-dependent protein kinase II isoforms as holoenzyme from porcine stomach

Gastric acid secretion is conveyed by different signal transduction pathways, among these being the muscarinic receptor M(3)-mediated acid secretion. There is some evidence that CaMkinase II is involved in the acetylcholine-conveyed acid release. The apparent CaMkinase II-isoenzymes gamma and delta were purified as a holoenzyme from homogenate of pig gastric mucosa to apparent homogeneity. The chromatographical steps comprised cationic exchanger chromatography, calmodulin affinity chromatography, anionic exchanger chromatography, and gel filtration. The CaMkinase II showed an apparent molecular mass of 332 +/- 17.3 kDa composed of 59- and 61-kDa subunits. The latter was characterized by a polyclonal antibody directed against CaMkinase II-delta. The purified CaMkinase II showed autophosphorylation and Ca(2+)/calmodulin-dependent activation (K(0. 5) = 5 nM). Moreover, the enzyme showed inhibition by the potent CaMkinase II inhibitor KN-62 in a dose-dependent manner. Addition of purified CaMkinase II inhibits the endogenous phosphorylation of a 105-kDa protein in the NaCl/Nonidet P-40 soluble fraction of the microsomal fraction of pig gastric mucosa. Our results suggest that CaMkinase II may regulate other protein kinases or phosphoprotein phosphatases, possibly by controlling acid production.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

Carbachol activates ERK2 in isolated gastric parietal cells via multiple signaling pathways

We previously reported that both carbachol and epidermal growth factor (EGF) are potent inducers of the extracellular signal-regulated protein kinases (ERKs) in isolated gastric canine parietal cells and that induction of these kinases leads to acute inhibitory and chronic stimulatory effects on gastric acid secretion. In this study we investigated the molecular mechanisms responsible for these effects. Both carbachol (100 microM) and EGF (10 nM) induced Ras activation. The role of Ras in ERK2 induction was examined by transfecting parietal cells with a vector expressing hemoagglutinin (HA)-tagged ERK2 (HA-ERK2) together with a dominantly expressed mutant (inactive) ras gene. HA-ERK2 activity was quantitated by in-gel kinase assays. Dominant negative Ras reduced carbachol induction of HA-ERK2 activity by 60% and completely inhibited the stimulatory effect of EGF. Since Ras activation requires the assembly of a multiprotein complex, we examined the effect of carbachol and EGF on tyrosyl phosphorylation of Shc and its association with Grb2 and the guanine nucleotide exchange factor Sos. Western blot analysis of anti-Shc immunoprecipitates with an anti-phosphotyrosine antibody demonstrated that both carbachol and EGF induced tyrosyl phosphorylation of a major 52-kDa shc isoform. Grb2 association with Shc was demonstrated by blotting Grb2 immunoprecipitates with an anti-Shc antibody. Probing of anti-Sos immunoprecipitates with an anti-Grb2 antibody revealed that Sos was constitutively bound to Grb2. To examine the functional role of Sos in ERK2 activation, we transfected parietal cells with the HA-ERK2 vector together with a dominantly expressed mutant (inactive) sos gene. Dominant negative Sos did not affect carbachol stimulation of HA-ERK2 but inhibited the stimulatory effect of EGF by 60%. We then investigated the role of betagamma-subunits in carbachol induction of HA-ERK2. Parietal cells were transfected with the HA-ERK2 vector together with a vector expressing the carboxy terminus of the beta-adrenergic receptor kinase 1, known to block signaling mediated by betagamma-subunits. In the presence of this vector, carbachol induction of HA-ERK2 was inhibited by 40%. Together these data suggest that, in the gastric parietal cells, carbachol activates the ERKs through Ras- and betagamma-dependent mechanisms that require guanine nucleotide exchange factors other than Sos.

L-365,260 inhibits in vitro acid secretion by interacting with a PKA pathway

The aim of this study was to analyse the antisecretory mechanism of L-365,260 in vitro in isolated rabbit gastric glands. We showed that compound L-365,260, described as a non-peptide specific competitive CCK-B receptor antagonist, was able to dose-dependently inhibit [14C]-aminopyrine accumulation induced by histamine (10(-4) M), carbachol (5x10(-5) M), 3-isobutyl-1-methyl-xanthine (IBMX) (5x10(-6) M) and forskolin (5x10(-7) M) with similar IC50 values respectively of 1.1+/-0.6x10(-7) M, 1.9+/-1.2x10(-7) M, 4.2+/-2.0x10(-7) M and 4.0+/-2.8x10(-7) M. We showed that L-365,260 acted beyond receptor activation and production of intracellular second messengers and that it had no action on the H+/K+ -ATPase. We found that L-365,260 inhibited cyclic AMP-induced [14C]-aminopyrine accumulation in digitonin-permeabilized rabbit gastric glands, suggesting that this compound acted, at least in part, as an inhibitor of the cyclic AMP-dependent protein kinase (PKA) pathway.

Lithocholyltaurine interacts with cholinergic receptors on dispersed chief cells from guinea pig stomach

Although bile acids damage gastric mucosa, the mechanisms underlying tissue injury induced by these agents are not well understood. To determine whether bile acids alter gastric secretory function, we investigated the actions of sodium cholate, deoxycholate, lithocholate, and their taurine and glycine conjugates on a highly homogeneous population of gastric chief cells. Lithocholyltaurine (LCT), a particularly injurious bile acid, caused a threefold increase in pepsinogen secretion (detectable with 100 nM and maximal with 10 microM LCT). When combined with other secretagogues, increasing concentrations of LCT caused progressive inhibition of carbamylcholine (carbachol)-induced pepsinogen secretion but did not alter CCK- or 8-bromo-cAMP-induced secretion. Taurine and unconjugated lithocholate did not alter basal or carbachol-induced secretion. These observations suggested that LCT is a partial cholinergic agonist. To test this hypothesis, we examined the actions of the cholinergic antagonist atropine on LCT-induced pepsinogen secretion. Atropine (10 microM) abolished carbachol- and LCT-induced pepsinogen secretion. Likewise, carbachol (0.1 mM) and LCT (1 mM) induced an atropine-sensitive, two- to threefold increase in cellular levels of inositol 1,4,5-trisphosphate. We examined the actions of LCT on binding of the cholinergic radioligand [N-methyl-3H]scopolamine ([3H]NMS) to chief cells. Half-maximal inhibition of [3H]NMS binding was observed with approximately 0.5 mM carbachol and 1 mM LCT. These results indicate that the bile acid LCT is a partial agonist for muscarinic cholinergic receptors on gastric chief cells.

Carbachol stimulation of gastric acid secretion and its effects on the parietal cell

1. The acid secretagogue effect of gastrin is mainly mediated by the release of enterochromaffin-like (ECL) cell histamine, but the mechanism of muscarinic stimulation of acid secretion remains unclear. The results of studying aminopyrine uptake in isolated parietal cells, and histamine release in isolated ECL cells suggest that muscarinic agents may act both directly on the parietal cell and indirectly via histamine release from ECL cells. 2. We examined parietal and ECL cell responses to the muscarinic agent carbamylcholine (carbachol) in conscious rats and in rat isolated vascularly perfused stomachs. 3. Intravenous carbachol stimulated acid secretion in conscious gastric fistula rats and increased H+K+ ATPase mRNA abundance, indicating activation of parietal cells. In these experiments there was no increase in portal venous histamine, or in oxyntic mucosal histidine decarboxylase (HDC) enzyme activity and HDC mRNA abundance. 4. In rat isolated stomachs stimulated with carbachol in the dose range 10 nM(-1) mM only the 1 microM concentration increased venous histamine significantly. 5. We concluded that the muscarinic agent carbachol stimulates acid secretion and H+K+ ATPase mRNA in vivo by a direct effect on the parietal cell, that does not depend on the release of ECL cell histamine.

Impaired gastric acid secretion in gastrin-deficient mice

To further understand the role of the peptide hormone gastrin in the development and function of the stomach, we have generated gastrin-deficient mice by gene targeting in embryonic stem cells. Mutant mice were viable and fertile, without obvious visible abnormalities. However, gastric function was severely affected by the loss of gastrin. Basal gastric acid secretion was abolished and could not be induced by histamine, carbachol, or gastrin. Histological analysis revealed alterations in the two cell types primarily involved in acid secretion, parietal and enterochromaffin-like (ECL) cells. Parietal cells were reduced in number with an accumulation of immature cells lacking H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase). ECL cells were positioned closer to the base of the gastric glands, with markedly lower expression of histidine decarboxylase. Gastrin administration for 6 days reversed the effects of the gastrin deficiency, leading to an increase in the number of mature, H(+)-K(+)-ATPase-positive parietal cells and a partial restoration of acid secretion. The results show that gastrin is critically important for the function of the acid secretory system.

Role of acetylcholine and gastrin-releasing peptide (GRP) in gastrin secretion

Using an isolated rat stomach infusion model, we investigated the role of gastrin-releasing peptide (GRP) and acetylcholine in the secretion of gastrin (which plays a major role in gastric acid secretion), and the relationship between gastrin secretion and stomach pH. Bombesin, which has a structure analogous to that of GRP, was used in the experiment. We also investigated whether acetylcholine has muscarine-like or nicotine-like action. Our findings pointed to the presence of an alternative, GRP-mediated, route for stimulating gastric secretion from G cells, other than the acetylcholine-mediated route. We injected bombesin to confirm the presence of such a GRP-mediated route; significantly increased gastrin secretion was observed, even under acidic conditions, in the gastric lumen, which has been considered to show almost no gastric secretion. This secretion was not inhibited by atropine. The results suggested that there are two routes for inducing gastrin secretion from G cells: an acetylcholine-mediated route and a GRP-mediated route (intramural peptide neurons). As GRP induced gastrin secretion, regardless of stomach pH, GRP was considered to be more closely related to gastrin secretion. The results also suggested that a muscarine-like action, particularly in the M3 receptor-mediated route, plays a significant role in acetylcholine-mediated gastrin secretion and that nicotine-like action is not involved in gastrin secretion.

G protein-coupled cholecystokinin-B/gastrin receptors are responsible for physiological cell growth of the stomach mucosa in vivo

Many peptide hormone and neurotransmitter receptors belonging to the seven membrane-spanning G protein-coupled receptor family have been shown to transmit ligand-dependent mitogenic signals in vitro. However, the physiological roles of the mitogenic activity through G protein-coupled receptors in vivo remain to be elucidated. Here we have generated G protein-coupled cholecystokinin (CCK)-B/gastrin receptor deficient-mice by gene targeting. The homozygous mice showed a remarkable atrophy of the gastric mucosa macroscopically, even in the presence of severe hypergastrinemia. The atrophy was due to a decrease in parietal cells and chromogranin A-positive enterochromaffin-like cells expressing the H+,K(+)-ATPase and histidine decarboxylase genes, respectively. Oral administration of a proton pump inhibitor, omeprazole, which induced hypertrophy of the gastric mucosa with hypergastrinemia in wild-type littermates, did not eliminate the gastric atrophy of the homozygotes. These results clearly demonstrated that the G protein-coupled CCK-B/gastrin receptor is essential for the physiological as well as pathological proliferation of gastric mucosal cells in vivo.

Localization of mRNA for the muscarinic M1 receptor in rat stomach

Cholinergic stimulation of receptors in the oxyntic mucosa results in secretion of mucus, pepsinogen and hydrochloric acid. There has been speculation as to the cellular localization of these receptors in the mucosa and as to which subtype is present in the different cell types. In the present study, utilizing radioactive riboprobes for the M1 muscarinic receptor subtype, we carried out in situ hybridization to determine which cells of the gastric corpus transcribe mRNA for this receptor. The antisense M1 probe hybridized strongly on the zymogen cells and, to a lesser extent, on the surface mucous cells and the muscle layers. Control sections from brain also displayed specific hybridization.

In vitro electro-pharmacological and autoradiographic analyses of muscarinic receptor subtypes in rat hypothalamic ventromedial nucleus: implications for cholinergic regulation of lordosis

Muscarinic agonists can act through the hypothalamic ventromedial nucleus (VMN) to facilitate lordosis. To elucidate the neuronal mechanism(s) underlying this muscarinic facilitation, effects of muscarinic agents on the single-unit activity of VMN neurons recorded in brain tissue slices of estrogen-primed female rats were analyzed. All the agonists tested, including acetylcholine (ACh), oxotremorine-M (OM), carbachol (CCh) and McN-A-343 (McN), evoked primarily excitation (80-100%), some inhibition (0-20%) and occasional biphasic responses (0-8%). By comparing the response magnitude and the effectiveness in evoking a response, the rank order for evoking excitation, the primary response, was found to be: OM > CCh > ACh approximately McN, which is consistent with that (OM > CCh > McN) for facilitating lordosis reported by others. This consistency and the frequency of its occurrence suggest that the excitatory electric action of the muscarinic agonists is related to their facilitatory behavioral effect. Experiments with antagonists selective for M1 (pirenzepine), M2 (AF-DX 116) and M3 (4-DAMP and p-F-HHSiD) indicate that muscarinic excitations are mediated by M1 and/or M3, but not M2. Since M1 receptors have been shown to be neither sufficient nor necessary to mediate the muscarinic facilitation, M3 receptor may be crucially involved in this behavioral effect. Autoradiographic assays of binding to [3H]4-DAMP with or without pirenzepine and AF-DX 116, also indicate the presence of M3 receptors in the VMN. Quantitative analyses show that the M3 binding was not affected by the in vivo estrogen priming required to permit muscarinic agonists to facilitate lordosis. Thus, while the excitation mediated by M3 is likely to be involved in muscarinic facilitation of lordosis, the regulation of M3 receptor density does not seem to be involved in the permissive

Pharmacological aspects of acid secretion

The secretion of gastric acid is regulated both centrally and peripherally. The finding that H2-receptor antagonists are able to reduce or abolish acid secretion due to vagal, gastrinergic, and histaminergic stimulation shows that histamine plays a pivotal role in stimulation of the parietal cell. In the rat, the fundic histamine is released from the ECL cell, in response to gastrin, acetylcholine, or epinephrine, and histamine release is inhibited by somatostatin or by the H3-receptor ligand, R-alpha-methyl histamine. The parietal cell has a muscarinic, M3, receptor responsible for [Ca]i regulation. Blockade of muscarinic receptors by atropine can be as effective as H2-receptor blockade in controlling acid secretion. However, general effects on muscarinic receptors elsewhere produce significant side effects. The different receptor pathways converge to stimulate the gastric H+,K(+)-ATPase, the pump responsible for acid secretion by the stomach. This enzyme is an alpha,beta heterodimer, present in cytoplasmic membrane vesicles of the resting cell and in the canaliculus of the stimulated cell. It has been shown that acid secretion by the pump depends on provision of K+Cl- efflux pathway becoming associated with the pump. As secretion occurs only in the canaliculus, this K+Cl- pathway is activated only when the pump inserts into the canalicular membrane. Transport by the enzyme involves reciprocal conformational changes in the cytoplasmic and extracytoplasmic domain. These result in changes in sidedness and affinity for H3O+ and K+, enabling active H+ for K+ exchange. The acid pump inhibitors of the substituted benzimidazole class, such as omeprazole, are concentrated in the canaliculus of the secreting parietal cell and are activated there to form sulfenamides. The omeprazole sulfenamide, for example, reacts covalently with two cysteines in the extracytoplasmic loops between the fifth and sixth transmembrane and the seventh and eighth transmembrane segments of the alpha subunit of the H+,K(+)-ATPase, forming disulfide derivatives. This inhibits ATP hydrolysis and H+ transport, resulting in effective, long-lasting regulation of acid secretion. Therefore, this class of acid pump inhibitor is significantly more effective and faster acting than the H2 receptor antagonists. K+ competitive antagonists bind to the M1 and M2 transmembrane segments of the alpha subunit of the acid pump and also abolish ATPase activity. These drugs should also be able to reduce acid secretion more effectively than receptor antagonists and provide shorter acting but complete inhibition of acid secretion.

Functional characterization of a muscarinic receptor stimulating gastrin release from rabbit antral G-cells in primary culture

In previous studies carbachol-induced stimulation of gastrin release from antral G-cells in primary culture suggested the presence of muscarinic acetylcholine receptors on this cell type. Therefore, we attempted to pharmacologically characterize the muscarinic acetylcholine receptor subtype involved. Enzymatically isolated rabbit antral mucosal cells (0.8% G-cells) were separated by counterflow elutriation yielding a fraction (1.7% G-cells) that was placed in culture on collagen-coated well plates. After 24-36 h of culture 13.0 +/- 2.4% of total adherent cells were immunoreactive for gastrin as shown by immunocytochemical staining using the avidin-biotin complex method. In this preparation basal gastrin release ranged from 3.3 +/- 0.3 to 4.1 +/- 0.3% of total cellular content. Maximal gastrin release in response to the acetylcholine receptor agonist carbachol (10(-4) M) or the selective muscarinic receptor agonist arecaidine propargyl ester (10(-4) M) was 8.5 +/- 0.4% and 7.6 +/- 0.4% of total cellular content, respectively. The EC50 values were 3.7 +/- 0.5 x 10(-6) M carbachol and 1.8 +/- 0.4 x 10(-6) M arecaidine propargyl ester. At a concentration of 10(-6) M the non-selective muscarinic receptor antagonist atropine and the muscarinic M3 receptor preferring antagonist hexahydro-sila-difenidol (HHSiD; M3 > or = M1 > M2) completely inhibited gastrin release in response to carbachol (Ki values: 52 x 10(-9) M atropine and 29 x 10(-9) M HHSiD) and arecaidine propargyl ester (Ki values: 11 x 10(-9) M atropine and 13 x 10(-9) M HHSiD).(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of the rabbit gastric CCK-A receptor

Cholecystokinin stimulates pancreatic zymogen secretion by binding with high affinity to a receptor on the pancreatic acinar cell. This receptor has been cloned and shown to be a CCK-A subtype. CCK also stimulates pepsinogen secretion from the gastric chief cell with high affinity. Using polymerase chain reaction with primers from the known sequence of the rat pancreatic CCK-A receptor cDNA, we prepared a 600 bp product from rat and rabbit stomach cDNA. From Southern analysis these represented a fragment of a gastric CCK-A receptor. PCR was then used to amplify a rabbit lambda ZAP II gastric epithelial cDNA library with the same primers, and the product was identified by sequencing as representing a CCK-A receptor fragment. When this PCR product was used to screen the library, ten positive clones were identified in a screening of 4.10(5) plaques, and several of these were sequenced. All had essentially the same sequence contained within 2 of these clones consisted of 427 amino acids and was 92% homologous (87% identity) to the known rat pancreatic CCK-A sequence but only 43% homologous to the gastric CCK-B sequence. The cDNA was subcloned into a pcDNA1 expression vector and transiently expressed in the human embryonic kidney cell line, HK 293. The responses of intracellular Ca2+ in these transfected cells to CCK and gastrin were monitored using video imaging. On the average 40% of the cells responded to CCK-8 by a transient elevation of [Ca2+]i followed by a steady state plateau. CCK was a high and gastrin a low affinity ligand for this signal, corresponding to the actions of these ligands on pepsinogen secretion from chief cells and somatostatin release from D cells. Hence from sequence and second messenger responses, the clone represents the CCK-A receptor presumably responsible for pepsinogen secretion by gastric chief cells and somatostatin release from gastric D cells.

Inhibitory effect of DS-4574, a mast cell stabilizer with peptidoleukotriene receptor antagonism, on gastric acid secretion in rats

We examined the inhibitory effect of DS-4574 (6-(2-cyclohexylethyl)[1,3,4]thiadiazolo[3,2-alpha]-1,2,3- triazolo[4,5-d] pyrimidin-9(3H)-one), a mast cell stabilizer with peptidoleukotriene receptor antagonism, on gastric acid secretion stimulated by several secretagogues in rats. In anesthetized rats with acute gastric fistulas, DS-4574 (50 mg/kg, intraduodenal) significantly inhibited gastric acid secretion induced by both carbachol (50 micrograms/kg, s.c.) and pentagastrin (75 micrograms/kg, s.c.) but not by histamine (2.5 mg/kg, s.c.). In unanesthetized pylorus-ligated rats, DS-4574 (10 and 25 mg/kg, intraduodenal) markedly suppressed increases in gastric acid output and histamine leakage into the gastric juice produced by carbachol (0.1 mg/kg, s.c.) or pentagastrin (1 mg/kg, s.c.). When the relationship between acid output and histamine leakage elicited by carbachol and pentagastrin was assessed, there was a close correlation (r = 0.84) that was highly significant (P < 0.01). In the in vitro study with rat gastric tissues, DS-4574 (10(-7)-10(-5) M) had no effect on the K(+)-dependent ATPase activity or on aminopyrine uptake into mucosal preparations containing parietal cells stimulated by carbachol (10(-5) M), histamine (10(-4) M), or dibutyryl-cyclic AMP (10(-3) M). These results suggest that the effect of DS-4574 may be mediated by inhibition of endogenous histamine from histamine-storing cells in the stomach.

Inhibition by Ro 31-8220 of acid secretory activity induced by carbachol indicates a stimulatory role for protein kinase C in the action of muscarinic agonists on isolated rat parietal cells

The bisindolylmaleimide Ro 31-8220 is a selective inhibitor of protein kinase C. This compound was used to investigate the involvement of protein kinase C in the stimulation of gastric acid secretion by the muscarinic cholinergic receptor on rat isolated parietal cells. The accumulation of the weak base aminopyrine by both crude and enriched preparations of parietal cells was used as an index of secretory activity. Ro 31-8220 antagonized (IC50, 1.0 microM) the effect of the activator of protein kinase C,12-O-tetradecanoylphorbol 13-acetate (TPA), on histamine-stimulated aminopyrine accumulation. Ro 31-8220 (0.1-2.14 microM) inhibited the aminopyrine response to 0.1 mM carbachol (IC50, 0.78 microM; 49% inhibition at 2.14 microM Ro 31-8220) and shifted the dose-response curve for the effect of carbachol concentration of aminopyrine accumulation downwards and to the right. No inhibition of aminopyrine accumulation induced by histamine was found with Ro 31-8220 (0.1-2.14 microM). In a preparation containing > 80% parietal cells incubated with 0.1 mM carbachol, 2.14 microM Ro 31-8220 inhibited aminopyrine accumulation by 43%, but had no effect on the increase in intracellular Ca2+ which was measured by using the fluorescent probe FURA-2. In conclusion, Ro 31-8220 (0.1-2.14 microM) produced a selective reduction in secretory activity in parietal cells by inhibition of protein kinase C. The predominant role of protein kinase C in parietal cells activated with carbachol is not to cause feedback inhibition of the response but to facilitate stimulation of secretory activity.